Wafer including an In-containing-compound semiconductor surface layer, and method for profiling its carrier concentration

ABSTRACT

Method for non-invasively profiling carrier concentration in In-containing compound semiconductor wafers that enables employing the profiled wafers themselves in semiconductor device applications. The method, which using the C/V technique profiles carrier concentration in wafers including an In-containing-compound semiconductor surface layer, is characterized in non-invasively profiling carrier concentration by contacting a liquid electrode on the wafer surface, and without using photo-etching, employing an applied voltage that is up to a voltage surpassing 10V.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to compound semiconductor wafers, and inparticular to the profiling of carrier concentration in In-containingcompound semiconductor wafers.

2. Description of the Background Art

Well-known methods of profiling carrier concentration withinsemiconductor wafers may be grossly divided into methods that measureHall coefficient, and the C/V technique, which measurescapacitance/voltage characteristics.

With the Hall-coefficient measuring methods, the carrier concentrationof the semiconductor wafer cannot be measured nondestructively since arectangular test piece for measuring must be cut out of a semiconductorwafer. This means that the carrier concentration of wafers themselvesinto which semiconductor devices are built cannot be profiled. Likewise,the Hall coefficient pertains to the test piece as a whole, and does notallow profiling of carrier concentration locally within the test piece.

In terms of the C/V technique, ordinarily a Schottky diode that is metalvapor-deposited onto the semiconductor wafer is formed, and an acvoltage of minute amplitude is superimposed onto a dc reverse-biasvoltage to measure the C/V characteristics. Being that the region withinthe semiconductor wafer on which the Schottky diode for measuring C/Vcharacteristics is formed cannot thereafter be employed forsemiconductor device formation, the routine C/V technique cannot be saidto be a non-invasive profiling method.

Likewise, with regard to compound semiconductor wafers, the routine C/Vtechnique is not a very attractive method. The reason why is that inrespect of compound semiconductor wafers, the barrier height of Schottkydiodes is low, oxide-film formation cannot be controlled, and further,problems such as chemical reactions between metals and the compoundsemiconductor can arise.

Therein, to profile carrier concentration in a compound semiconductorwafer, electrochemical C/V techniques utilizing an electrolyte as anelectrode have been used. (See for example, J. Electrochem. Soc., Vol.133, 1986, pp. 2278–2283.)

Reference is made to FIG. 2, a block diagram schematically illustratinga conventional electrochemical C/V technique. In the electrochemical C/Vanalyzer set out in FIG. 2, the interior of a cell 1 is filled with anelectrolyte 2 such as an aqueous HCl solution. A calomel electrode 3 asa reference electrode is inserted into the cell 1. The cell 1 has aring-shaped opening 1 a, and a compound semiconductor wafer 4 whose C/Vcharacteristics are to be measured is contacted with the electrolyte 2via the opening 1 a, wherein the electrolyte 2 acts as one of theelectrodes. A probe electrode 5 as the other electrode is contacted onthe compound semiconductor wafer 4. An electrical analyzing unit 6supplies a dc reverse-bias voltage and around a 3000-Hz ac superimposedvoltage to the reference electrode 3 and the probe electrode 5 tomeasure the C/V characteristics.

In a depth w from the surface where the compound semiconductor wafer 4is in contact with the electrolyte electrode 2, the carrierconcentration N(cm⁻³) within the wafer may be determined using thefollowing formula (1).N(w)=(−C ³ /qεA ²)(dC/dV)⁻¹  (1)

Herein, w expresses the depth from the wafer surface to the edge of thedepletion layer. That is, N(w) expresses the carrier concentration N inthe depth w from the wafer surface. Likewise, C expresses capacitancemeasured by a dc reverse-bias voltage; q, electronic charge; ε,permittivity; A, measurement area; and dC, variation in capacitancedepending on variation dV in the superimposed ac voltage. Here, thedepth w may be found from the following formula (2).w=∈A/C   (2)

In terms of an electrochemical C/V technique utilizing an electrolyteelectrode as described above, profiling carrier concentration throughdepths of more than 3 μm is difficult unless a reverse-bias voltage thatexceeds 10 V is applied. However, wherein an electrolyte electrode suchas, e.g., an aqueous HCl solution is utilized, electrical breakdown ofthe electrolyte sets in when a high reverse-bias voltage in excess of 10V is applied, giving rise to problems in that bubbles of hydrogen andoxygen cling to the wafer surface and make it impossible to measure theC/V characteristics. Applying too high a voltage can also give rise toproblems in that the leakage current grows large, and electrolyteleakage occurs.

Consequently, in conventional electrochemical C/V analyzers, the maximumvalue of the reverse-bias applied voltage is in general limited to 10 V.To work around this limitation, carrier concentration through depths ofmore than 3 μm is profiled by repeating C/V analysis using an appliedvoltage of under 10 V, and wafer surface etching using a photo-etchingprocess.

This means that, as shown in FIG. 2, the cell 1 in a conventionalelectrochemical C/V analyzer is furnished with a light-receiving window1 b. By shining light 7 onto the wafer surface where it contacts theelectrolyte 2 at the ring-shaped opening 1 a, the electrolyte 2 works asan etchant and the wafer surface is removed to a predetermined depth byphoto-etching. Then a succeeding C/V analysis is performed with thesurface freshly formed by the etching as a new reference.

With this method of profiling carrier concentration by repeating C/Vanalysis and photo-etching in this way, time is required for theetching; for a C/V analysis in order to profilecarrier-concentration/distribution about 2 μm in the depth direction,around 1 hour is necessary. Furthermore, being that the photo-etchedregion cannot thereafter be employed for semiconductor device formation,the conventional electrochemical C/V technique cannot be said to be anon-invasive profiling method.

Moreover, In-containing compound semiconductor wafers for formingoptical-communications photo detectors must be furnished with anepitaxial layer 5 to 8 μm in thickness, and if the carrier concentrationwithin an epitaxial layer that thick is profiled by the repeating of C/Vanalysis and photo-etching processes, the C/V analysis alone ends uptaking around 3 to 4 hours. Still further, performing photo-etching withconsistency is not easy, and gaining high precision in C/V analysis isdifficult.

SUMMARY OF INVENTION

Taking the above-described issues in the prior art into consideration,an object of the present invention is to develop a method ofnon-invasively profiling carrier concentration/distribution in a shorttime to a depth of several μm within an In-containing compoundsemiconductor wafer, and at the same time to provide the profiled waferitself for use directly in device fabrication.

A method as defined by one mode of the present invention, whichemploying the C/V technique profiles carrier concentration in a waferincluding an In-containing-compound semiconductor surface layer, ischaracterized in non-invasively profiling carrier concentration bycontacting a liquid electrode on the wafer surface, and without using aphoto etching process, employing an applied voltage that is up to avoltage surpassing 10V.

Here, an aqueous EDTA solution is preferably utilized as the liquidelectrode. Likewise, the aqueous EDTA solution preferably contains 80%or more EDTA. Furthermore, liquid tiron or a metal gallium (Ga) melt maybe utilized as the liquid electrode.

A method as defined by one further mode of the present invention, whichemploying the C/V technique profiles carrier concentration in a waferincluding an In-containing-compound semiconductor surface layer, ischaracterized in contacting a metal Ga melt on the wafer surface, thensolidifying the metal Ga melt to form a metal Ga electrode, employing anapplied voltage that is up to a voltage surpassing 10V to profilecarrier concentration, and clearing away the metal Ga electrode afterthe profiling.

Another mode as defined by the present invention is characterized inthat carrier concentration in a wafer including anIn-containing-compound semiconductor surface layer is non-invasivelyprofiled, and in that the wafer can be employed as it is, after itscarrier concentration has been non-invasively profiled, for use indevice processing. Here, this wafer may be one that has been profiled byany of the above-described non-invasive carrier-concentration profilingtechniques.

In accordance with the present invention, carrierconcentration/distribution within an In-containing compoundsemiconductor wafer may be non-invasively profiled to a depth of severalμm in a short time and with a high degree of accuracy, and the profiledwafer itself may be provided for use directly in device fabrication.

From the following detailed description in conjunction with theaccompanying drawings, the foregoing and other objects, features,aspects and advantages of the present invention will become readilyapparent to those skilled in the art.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph plotting results of profiling, according to one modeof embodying the present invention, carrier concentration within acompound semiconductor wafer; and

FIG. 2 is a schematic block diagram representing one example of aconventional C/V analyzer.

DETAILED DESCRIPTION

Embodiment 1

In a carrier concentration profiling method under a first embodiment, animproved electrochemical C/V technique is employed. A rebuilt version ofthe device in FIG. 2 may be utilized as the C/V analyzer for theimproved electrochemical C/V technique. In particular, the electricalanalyzing unit 6 was reformed to enable it to apply in excess of 10V andup to 60V as a reverse-bias applied voltage. Further, inasmuch asphoto-etching is unnecessary in the present invention, thelight-receiving window 1 b on the cell 1 is not required.

An aqueous solution of EDTA (ethylene diamine tetra-acetic acid) isutilized as an electrolyte 2′. The electrolyte 2′ preferably contains 80or more percent by mass EDTA. The reason why is because electrolysis ofthe water is thereby prevented when high voltage has been applied.

The carrier concentration/distribution in a compound semiconductor waferutilized in fabricating optical communications devices was actuallyprofiled in accordance with the present embodiment. The compoundsemiconductor wafer included, epitaxially grown in turn on an indiumphosphate (InP) wafer: an InP layer of approximately 1 μm thickness, anindium-gallium arsenide (InGaAs) layer of approximately 3 μm thickness,and an InP layer of approximately 2 μm thickness.

The epitaxial layers contained up to 1×10¹⁶ cm⁻³ dopant throughout. TheFIG. 1 graph plots the results of profiling the carrierconcentration/distribution in this compound semiconductor wafer. In thegraph, from the surface of the wafer up to the depth of about 1 μm thereis no curved line showing carrier concentration, because there is asurface depletion layer through that depth.

In particular, the horizontal scale in the FIG. 1 graph indicates thedepth w(μm) from the surface of the epitaxial layers on, to the edge ofthe depletion layer in, the compound semiconductor wafer; while thevertical scale indicates the log₁₀ (common logarithm) of the carrierconcentration N(cm³) through the depth w. From this graph it may beconfirmed that any one of the epitaxial layers had a carrierconcentration of under 1×10¹⁶ cm⁻³. It may likewise be confirmed thatcarrier concentration peaks are formed in positions corresponding to thelocations of heterojunction interfaces that are at 2 μm and 5 μm fromthe wafer surface.

The profiling data in FIG. 1 was obtainable with an approximately10-minute measurement. Furthermore, the FIG. 1 data is about the same aswhat would take 3 hours to profile while photo-etching using an aqueousHCl solution. On the other hand, inasmuch as photo-etching is not neededwith the present invention, incidents of etching unevenness that requirechanging the measuring locale do not occur. What is more, profilingerrors due to etching unevenness can be eliminated.

It should be noted that in terms of the present embodiment, despite notphoto-irradiating, slight etching on the order of 0.1 nm thickness wasactually observed. Nevertheless, device evaluation confirmed that inactual practice slight etching in this way does not affect deviceprocessing. In short, a carrier concentration profiling method accordingto the present invention is non-invasive, whereby directly profiledwafers can themselves serve in device fabrication.

Embodiment 2

Embodiment 2 is similar to Embodiment 1, except in differing fromEmbodiment 1 only in that the electrolyte electrode was changed from anaqueous EDTA solution to an aqueous tiron solution. (Tiron has themolecular formula: C₆H₂(OH)₂(SO₃Na)₂.H₂O.)

Carrier concentration within GaAs compound semiconductor wafers has inthe conventional art been profiled utilizing an electrode of aqueoustiron solution while photo-etching. However, because InP-compoundsemiconductor wafers cannot be photo-etched with aqueous tiron solution,aqueous tiron solution has not been utilized for profiling carrierconcentration within InP-compound semiconductor wafers. Nevertheless,the present inventors, by exploiting in reverse the fact thatInP-compound semiconductor wafers are not photo-etched in aqueous tironsolution, succeeded in non-invasively profiling carrier concentration inInP-compound semiconductor wafers to great depths from the surface.

Embodiment 3

Embodiment 3 is similar to Embodiments 1 and 2, except in differing fromEmbodiments 1 and 2 only in that the electrolyte electrode was changedto a metal Ga-melt electrode.

Inasmuch as its melting point is an extremely low 29° C., Ga may beemployed as a liquid electrode that does not produce effects on compoundsemiconductor wafers. Here, although the use of mercury as aliquid-metal electrode is conceivable, being that its toxicity ishazardous to human beings, and that from a safety viewpoint handlingproblems are liable to arise, mercury cannot be said to be desirable asa liquid electrode.

Embodiment 4

Embodiment 4 is similar to Embodiment 3 in that metal Ga is employed asan electrode. In Embodiment 4, however, a cell 1 like that illustratedin FIG. 2 is made unnecessary. In the carrier-concentration profilingmethod of Embodiment 4, to begin with a metal-Ga melt is soaked into asponge-like retaining material. The retaining material is then contactedon the surface of a compound semiconductor wafer. Thus the wafer surfaceis wetted by and put in contact with the metal-Ga melt soaked into thesponge-like retaining material. The metal-Ga melt in this situation isthen solidified, forming on the compound semiconductor wafer surface asolid metal-Ga electrode running along the sponge-like retainingmaterial.

After utilizing the solid metal-Ga electrode to profile carrierconcentration within the compound semiconductor wafer, the solidmetal-Ga electrode is made molten once more to clear it from the wafersurface. The wafer from which the metal-Ga electrode has been cleared isitself then made available for device fabrication. In short, in terms ofthis embodiment as well, carrier concentration in a compoundsemiconductor wafer may be non-invasively profiled.

Only selected embodiments have been chosen to illustrate the presentinvention. To those skilled in the art, however, it will be apparentfrom the foregoing disclosure that various changes and modifications canbe made herein without departing from the scope of the invention asdefined in the appended claims. Furthermore, the foregoing descriptionof the embodiments according to the present invention is provided forillustration only, and not for limiting the invention as defined by theappended claims and their equivalents.

1. A non-invasive, semiconductor-wafer carrier concentration profilingmethod, comprising steps of: equipping a C/V analyzer with alight-receiving-windowless electrolyte cell having an opening forelectrolyte-wafer contact; filling the cell with a liquid electrode;preparing a semiconductor wafer composed of an In-containing compoundand superficially onto which at least one In-containing-compoundsemiconductor layer has been heteroepitaxially grown; placing said waferon said opening in the electrolyte cell so as to put the liquid insuperficial contact with the wafer to allow the liquid to function as anelectrode; and employing applied voltages, including at least a maximumvoltage that surpasses 10V but is not greater than about 60V, to profilethe wafer's C/V characteristics.
 2. The carrier concentration profitingmethod set forth in claim 1, wherein an aqueous EDTA solution isutilized as the liquid electrode.
 3. The carrier concentration profilingmethod set forth in claim 2, wherein the aqueous EDTA solution contains80% or more EDTA.
 4. The carrier concentration profiling method setforth in claim 1, wherein liquid tiron is utilized as the liquidelectrode.
 5. The carrier concentration profiling method set forth inclaim 1, wherein a metal Ga melt is utilized as the liquid electrode.